The development in recent years of copying devices such as full-color copiers has enhanced the quality of reproduction to a level at which one cannot distinguish the copy from the original with the naked eye. Such faithful reproductions can now be obtained inexpensively. However, along with this benefit comes the increased risk of copiers being put to such illicit uses as counterfeiting money, negotiable securities, and other original documents whose reproduction is legally prohibited, or the purloining of top secret documents by copying them. To avert this danger, various devices have been developed to prevent copiers from being used for counterfeiting. One such device is the image processing device disclosed in Japanese Patent Publication 2-210591.
That image processing device is installed in a full-color digital copy machine which reads the original image placed on the glass of the copier by scanning the entire original four times and executes the processing needed to copy it. Let us assume a bank note has been placed on the glass of the copy machine. On the first scan, the device detects the rough location of watermark B, the mark on bank note A which identifies it as money (see FIG. 37), the item which the machine is to be prevented from copying, on the assumption that the document on the glass is, in fact, a bank note.
On the second scan, the device uses the location of watermark B to detect the exact position of the bill and the angle at which it is placed (i.e., the exact position coordinates of the bill on the glass). Because the length, size and shape of bank note A, the item to be detected, are already known, we can extract the edge C (the hatched portion) of bank note A by binarizing the image data at a given threshold. Using this edge, we can obtain the coordinates (x.sub.1, y.sub.1) and (x.sub.2, Y.sub.2) of two apices D.
In this way we can obtain the slope theta at which the bill is oriented. On the third scan, we use the exact position of the bill (the coordinates of apices D and angle theta) obtained in the second scan to calculate the position coordinates (x.sub.3, y.sub.3) of the red seal E stamped on the bill. Based on these position coordinates, we extract an image from the region in which the seal is placed, and we make a judgment as to whether there is in fact a red seal on this item.
By scanning the item three times, the device is able to determine whether or not a document which it is supposed to detect (in this case, a bank note) is loaded in the copier. If the device detects that a prohibited document such as a bank note has been loaded to be copied, it will perform its specified anti-counterfeiting operations on the fourth scan, such as making the entire display go black or preventing the copier from operating.
The existing device described above requires an extremely large memory capacity merely to store data concerning the size of paper money. Inevitably, then, only a limited number of kinds of money can be detected.
Furthermore, three scans are needed to determine whether the item on the glass is a non-reproducible document such as a bank note. If the apices D of bank note A are folded down or mutilated in some other way, the device will be unable to determine the location of red seal E, and consequently will be unable to detect that the item being processed is a bank note, one of the items it is supposed to detect.
One solution to this problem would be to surround specified pattern E with a guideline F consisting of either a heavy continuous line or a broken line, as shown in FIGS. 38 (A) and (B). The region inside guideline F could then be extracted to determine if it contains the specified pattern. If the type of guideline F shown in the drawing is used, then, it will be easier to separate the region containing the specified pattern from the other image data. The straight portions of guideline F can be used to calculate the slope and angle of orientation of the bill as well as the coordinates of the apices. The use of a guideline thus allows us to extract the specified pattern more easily and accurately.
When this method is used, however, guideline F is conspicuous on the bill (because it is a large feature). It can easily be seen and recognized, as can the specified pattern E which it encloses. This makes it easier for someone to tamper with guideline F or specified pattern E (by adding extra lines or erasing portions of lines). If the pattern or guideline has been tampered with, it is possible that the pattern will not be recognized (or extracted), or that even if it is recognized it will not be determined to be the right pattern.
Another problem is that at least two scans are required to detect the location of bank note A and guideline F, find the location of the specified pattern, extract the pattern and determine whether it is the right pattern. The device could not then be used in copy machines which read the image in a single scan. If we wish to extract the specified pattern without any preliminary scans, our only recourse is to match the pattern against a reference as each pixel is read in the raster direction. Since we do not know the orientation of the pattern (i.e., the angle at which the image is oriented on the glass), we must also consider the angle of rotation when we match the pattern. This makes our task extremely difficult.
The existing method or the method using guideline F required selecting from the many patterns on a non-reproducible document a pattern suitable for feature analysis (i.e., one which can be discriminated accurately and which is not found on any document which may be legally copied), such as the red seal on a bank note, and matching it against a reference pattern. Even if we limit our focus to Japan, there are many, many documents which may not legally be copied. To detect all of them reliably would require a huge memory capacity and lengthy processing. Existing devices, then, lack the capacity to execute the required processing in real time. Furthermore, such devices require three scans to determine that the document may not be copied. This too stands in the way of high-speed processing.
If, after the copier is manufactured, a new document appears which may not be copied, the device will not be able to handle the document or even detect it. To detect the document, a new set of features must be selected and taught to the copier, which is a complicated procedure. If we are to include the documents of other countries, the problem becomes even more overwhelming.